1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a membrane electrode assembly and a separator in a stacking direction. The membrane electrode assembly includes a pair of electrodes, and an electrolyte interposed between the electrodes. A reactant flow field is formed in the fuel cell for supplying a reactant gas along an electrode surface.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (electrolyte electrode assembly) which includes an anode, a cathode, and a solid polymer electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is sandwiched between separators to form a power generation cell (unit cell). In use on a vehicle, several tens to hundreds of the unit cells are stacked together to form a fuel cell stack.
In the fuel cell, so called the internal manifold structure is often adopted for supplying a fuel gas and an oxygen-containing gas as reactant gases to an anode and a cathode of each of the stacked power generation cells. The internal manifold includes reactant gas supply passages and reactant gas discharge passages extending through the power generation cells in the stacking direction. Inlets and outlets of reactant gas flow fields for supplying reactant gases along electrode surfaces are connected to the reactant gas supply passages and the reactant gas discharge passages, respectively.
In the structure, the opening areas of the reactant gas supply passages and the reactant gas discharge passages are relatively small. Therefore, in order to allow the reactant gases to flow smoothly, buffers for dispersing the reactant gases need to be required adjacent to the reactant gas supply passages and the reactant gas discharge passages. For example, in a solid electrolyte fuel cell disclosed in Japanese Laid-Open Patent Publication No. 06-140056, a separator 1 as shown in FIG. 5 is provided.
Supply/discharge holes 2a as passages of one of the reactant gases and supply/discharge holes 2b as passages of the other of the reactant gases are provided along diagonal lines, at four corners of the separator 1. Ridges and grooves are provided alternately on a surface 1a of the separator 1 to form a reactant gas flow field 3a. Likewise, a reactant gas flow field 3b is formed on a surface 1b of the separator 1.
On the surface 1a of the separator 1, the supply/discharge holes 2a and the reactant gas flow field 3a are connected through gas distribution channels (buffers) 4a, and a plurality of current collectors 5 are provided in the gas distribution channels 4a. On the surface 1b, gas distribution channels 4b connecting the supply/discharge holes 2b and the reactant gas flow field 3b are formed, and a plurality of current collectors 5 are provided in the gas distribution channels 4b. 
In the conventional technique, the diameter in each opening of the supply/discharge holes 2a is significantly small in comparison with the width of the reactant gas flow field 3a (in the direction indicated by the arrow X). Thus, it is not possible to uniformly supply the reactant gas through the gas distribution channels 4a along the width of the reactant gas flow field 3a. 
In the structure, in some locations in the power generation area of the reactant gas flow field 3a, the flow rate of the reactant gas tends to be small. Thus, when the load is small, power generation cannot be performed stably due to the water remaining in the fuel cell. When the load is large, concentration overpotential occurs due to shortage of the reactant gas, and the desired power generation cannot be achieved.